The present invention relates to a rotary electromechanical device used in a field in which stringent requirements are imposed with respect to torque characteristics and for small torque ripples, as well as to a pulley driving system using the rotary electromechanical device.
In a rotary electromechanical device like an induction motor, for example, grooves, or so-called slots, for accommodating coils are formed in the proximity of an air gap by punching an iron core in a stator or a rotor, and there occur areas of high and low magnetic flux densities in a magnetic field formed in the air gap due to a difference in reluctance between the slots and the iron core. For this reason, magnetic fluxes formed in the air gap contain a lot of harmonic components and, therefore, there occur torque variations in the induction motor. The torque variations occurring in the induction motor are referred to as torque ripples, and it is necessary to solve this torque ripple problem.
Conventionally, so-called skewed slots, which are the slots formed in the stator or the rotor at an oblique angle to the direction of an axis of rotation, have been employed to solve this problem. Another approach to the solution of the problem is disclosed in Japanese Laid-open Utility Model Publication No. 4-97459, in which a rotary electromechanical device comprises a plurality of stators which share a single rotor and the positions of slots formed in the multiple stators are displaced from one another in the direction of rotation, or a rotary electromechanical device comprises a plurality of rotors which share a single stator and the positions of slots formed in the multiple rotors are displaced from one another in the direction of rotation, and coils accommodated in the slots are connected in series.
Stringent requirements are imposed on a rotary electromechanical device used in an elevator, for example, for small torque ripple as well as concerning the amount of torque. The rotary electromechanical device employing the aforementioned skewed slots and the rotary electromechanical device disclosed in Japanese Laid-open Utility Model Publication No. 4-97459, however, has a problem that it is difficult to obtain the amount of torque satisfying the aforementioned requirements due to a reduction in facing surface areas of magnetic poles of the rotors and magnetic poles of the stators.
In particular, a so-called magnetic pole concentrated winding type rotary electromechanical device, in which coils are wound closely around teeth formed on a core of a stator, has a problem that a reduction in the amount of torque increases if a conventional torque ripple reduction method as stated above is used.
The invention has been made to solve the to aforementioned problems and, thus, it is an object of the invention to provide a rotary electromechanical device which can prevent a reduction in the amount of torque and reduce torque ripples as well as a pulley driving system employing such a rotary electromechanical device.
According to the invention, a rotary electromechanical device comprises a rotary shaft, a plurality of motor sections, each including a rotor and a stator positioned to face each other, the rotors of the individual motor sections being fixedly mounted in series on the rotary shaft, and power supplies for driving the multiple motor sections independently of one another, wherein the relative distance between magnetic poles of the same polarity on the rotor and the stator in the circumferential direction of the rotary shaft differs between the individual motor sections. This construction makes it possible to provide a high-performance rotary electromechanical device capable of suppressing torque ripples without reducing the amount of torque.
In one form of the invention, the multiple motor sections are two motor sections driven by the respective power supplies of a three-phase alternating current type, and the difference between the relative distances between the motor sections corresponds to a phase difference equal to an odd multiple of 90xc2x0 in terms of electrical angle. This arrangement is advantageous in that it can cancel out two types of torque ripples, that is, the torque ripples (whose frequency is six times the power supply frequency) occurring due to harmonic magnetic fluxes and the torque ripples (whose frequency is twice the power supply frequency) occurring due to manufacturing errors of the stators (deformation of surfaces facing the rotors).
In another form of the invention, the stators are of a magnetic pole concentrated winding type in which coils are wound closely around individual teeth formed on stator cores of the individual stators. Compared to the conventional concentrated winding type motors whose torque is noticeably decreased due to the torque ripple reduction method, the aforementioned arrangement of the invention can suppress the torque ripples without reducing the amount of torque. In addition, the arrangement of the invention helps improve productivity.
In still another form of the invention, the teeth formed on each of the stator cores are separated from one another or the teeth are joined in such a manner that intervals between winding portions of the adjacent teeth on which coils are wound can be increased. This arrangement is advantageous in improving the efficiency of wire-winding operation.
In yet another form of the invention, the motor sections are of a radial gap type in which facing surfaces of the stators and the rotors are arranged parallel to the axial direction of the rotary shaft. This arrangement is advantageous in that it can cancel out electromagnetic forces exerted between the stators and the rotors, reduce forces exerted on a bearing and prolong its service life.
In another form of the invention, the motor sections are of an axial gap type in which facing surfaces of the stators and the rotors are arranged at right angles to the axial direction of the rotary shaft. This arrangement mares it possible to obtain a greater mechanical accuracy of the facing surfaces of the stators and the rotors and reduce torque ripples occurring in relation to manufacturing errors.
In one variation of the aforementioned radial gap type arrangement, the magnetic poles provided on the stators or the magnetic poles provided on the rotors, or the magnetic poles provided on both the stators and the rotors, are arranged at an oblique angle to the axial direction of the rotary shaft. This arrangement makes it possible to reduce the torque ripples in a more reliable fashion. In addition, it is made possible to cancel out torque ripples related to various components of the power supply frequency as well as components other than the power supply frequency when such components occur.
In one variation of the aforementioned axial gap type arrangement, the magnetic poles provided on the stators or the magnetic poles provided on the rotors, or the magnetic poles provided on both the stators and the rotors, are arranged at an oblique angle to the radial direction of rotary shaft. This arrangement also makes it possible to reduce the torque ripples in a more reliable fashion. In addition, it is made possible to cancel out torque ripples related to various components of the power supply frequency as well as components other than the power supply frequency when such components occur.
According to the invention, a pulley driving system comprises a rotary shaft, a plurality of motor sections, each including a rotor and a stator positioned to face each other, the rotors of the individual motor sections being fixedly mounted in series on the rotary shaft, a pulley section arranged in series with the rotors in the axial direction of the rotary shaft and fixedly mounted on the rotary shaft, and power supplies for driving the multiple motor sections independently of one another, wherein the relative distance between magnetic poles of the same polarity on the rotor and the stator in the circumferential direction of the rotary shaft differs between the individual motor sections. This construction makes it possible to provide a high-performance pulley driving system capable of suppressing torque ripples without reducing the amount of torque.
In one form of the invention, iron cores of the individual rotors and the pulley section are formed into a single structure. This construction makes it possible to increase the rigidity of individual joints and exactly invert the phases of the torque ripples, so that the torque ripples can be suppressed in a reliable fashion.
In another form of the invention, the whole of the multiple motor sections are provided on one side of the pulley section in the axial direction of the rotary shaft. This arrangement makes it possible to assemble individual elements from one side of the system, resulting in an improvement in productivity. In addition, since the distance between the motor sections is reduced, the rigidity of a portion between them increases and alignment of the phases of the torque ripples becomes even easier. Furthermore, it is possible to effectively use internal spaces of the stators in the case of motors having a large diameter.
In still another form of the invention, the multiple motor sections are provided on both sides of the pulley section in the axial direction of the rotary shaft. This arrangement makes it possible to balance forces exerted on the pulley section and distribute forces exerted on the pulley section and the motor sections and, therefore, there is no need to unnecessarily increase the mechanical strength of connecting parts between the pulley section and the motor sections and the physical size of the system can be reduced.
These and other objects, features and advantages of the invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings.